1. Field of the Invention
The present invention relates to a method and apparatus for manufacturing semiconductor devices having crystal structures with three-dimensional periodical refractive index distribution.
2. Related Background Art
Photonic crystals are specific semiconductor devices having therein a three-dimensional (3D) periodical refractive index distribution structure, which are expected for use as new optical material. With such crystals, a band structure (photonic band) is formed with respect to photon energy in a way corresponding to an electron energy band structure in solid-state crystals. Due to this, photon""s energy state that can exist within a photonic crystal is limited, which in turn makes it possible to achieve a wide variety of light controllabilities which have been considered impossible until today, including freely curving light rays with no losses, controlling spontaneous emission light, and the like.
3D structures of the photonic crystals stated above include an asymmetrical face-centered cubic (FCC) structure with asymmetry introduced into standard FCC structures, a diamond structure, and others. Furthermore, a variety of applications are possible, including but not limited to fabrication of a waveguide path through formation of a linear array of defects into these photonic crystals, controlling output light with a light emitting element embedded therein. In view of the applicability to the optical devices, several preferable conditions for these crystals may typically include: (1) materials used are semiconductor-based ones; (2) it is easy to embed the light emitting element in the crystals; (3) it is easy to form defects (disorder of refractive index); and, (4) a flow of current is possible.
Photonic crystals are manufacturable by several proposed methods including methods by means of self-ordering such as an opal method and a self-cloning method, and a three-direction semiconductor dry etching method. The opal method is for fabricating a 3D crystal structure while letting semiconductor microparticles such as silicon microparticles settle to the bottom in a suspension liquid. This approach is capable of obtaining FCC structures. However, it is difficult to create complete band gaps in this method. Also, formation of the defects is difficult.
The self-cloning method is capable of obtaining simple cubic crystal structures. However, it is difficult to create complete band gaps as in the opal method. And the defects can form only in a single direction.
The three-direction semiconductor dry etching method is for forming 3D structures by use of semiconductor anisotropic etching technique, and capable of obtaining asymmetrical FCC structures. However, high aspect ratios are required in this method, and it is very difficult to fabricate them with sufficient accuracy. Also formation of the defects is difficult.
One approach to avoid these difficulties is to employ a manufacturing method using precision multilayer stack technologies based on wafer fusion (wafer bonding) technique. This method includes the steps, for example, of preparing wafers each having a semiconductor/air diffraction grating layer (lattice layer, two-dimensional structure) as formed on a semiconductor substrate, and then stacking and fusing these wafers"" lattice layers together while establishing predetermined positional relations (such as mutually forming angles, periods, etc.) between them, thereby obtaining periodical 3D structures such as an asymmetric FCC structure.
In the above-stated fusion multilayer stack method, it should be required that respective wafers (on-wafer lattice layers) be precisely aligned in position with each other. More specifically, in order to obtain the photonic band, the positional relation between the lattice layers must be accurately determined in stacking the lattice layers, e.g. stacking two lattice layers over each other with a structure period thereof being positionally shifted by a half period.
However, it is not easy to accomplish such precision positional alignment between separate on-wafer lattice layers. Thus, there arise the problems including the problem that any sufficient performances of photonic crystals are hardly obtainable, and the problem that an increased length of time is required for such position alignment process in the manufacture of the photonic crystals.
The present invention has been made in view of the problems stated above, and its object is to provide an improved method and apparatus for manufacturing a semiconductor device having 3D periodical refractive index distribution using precision multilayer stack technique by means of a wafer fusion, which are capable of achieving precision alignment in position between lattice layers being stacked over each other while reducing complexity in position alignment procedure thereof.
To attain the foregoing object, a semiconductor device manufacturing method in accordance with the present invention is a method for manufacturing a semiconductor device having a crystal structure with three-dimensional (3D) periodical refractive index distribution, characterized in that said method comprises: (1) a wafer fabrication step of forming two semiconductor wafers each having one surface with at least one lattice layer being formed thereon, the lattice layer having a two-dimensional structure including a crystal lattice region as formed with a crystal lattice period and an adjustment lattice region with an adjustment lattice period greater than the crystal lattice period; (2) a wafer hold step of integrally holding the two semiconductor wafers with the lattice layers facing each other; (3) a first alignment step of irradiating observation light onto the two semiconductor wafers being held integrally for detection of a lattice image obtained from the adjustment lattice region to thereby perform rough alignment between the semiconductor wafers; (4) a second alignment step of irradiating laser light onto the two semiconductor wafers with rough alignment applied thereto for detection of a diffraction image obtained from the crystal lattice region to thereby perform fine alignment between the semiconductor wafers; and (5) a wafer fusion step of fusing the two semiconductor wafers after completion of the fine alignment while letting the lattice layers facing each other be stacked over each other.
In addition, a semiconductor device manufacturing apparatus of the invention is an apparatus for manufacturing a semiconductor device having a crystal structure with 3D periodical refractive index distribution, characterized in that said apparatus comprises: (a) wafer hold means for integrally holding two semiconductor wafers each having one surface with at least one lattice layer having a two-dimensional structure being formed thereon with the lattice layers facing each other; (b) a first light source for irradiating observation light onto the two semiconductor wafers being held integrally; (c) first detection means for detecting a lattice image obtained by the observation light to thereby perform rough alignment between the semiconductor wafers; (d) a second light source for irradiating laser light onto the two semiconductor wafers being held integrally; and (e) second detection means for detecting a diffraction image obtained by the laser light to thereby perform fine alignment between the semiconductor wafers.
With the manufacturing method and apparatus stated above, a diffraction image as formed by a multilayer lattice structure upon irradiation of laser light is used to perform position alignment between lattice layers of two semiconductor wafers being stacked and fused together. More specifically, each lattice layer is expected to function as a diffraction grating upon irradiation of the laser light. At this time, each diffraction light spot in the diffraction image being detected, e.g. first-order diffraction light spot, will behave to change in intensity due to mutual relation in position between stacked lattice layers (such as shift in lattice period). Thus, it is possible by utilizing this intensity change to achieve precise position alignment therebetween.
Another important feature of the invention is that prior to the position alignment using the laser light diffraction image, rough position alignment is carried out by use of a lattice image obtained by the observation light using normal light without coherence. While the laser light assisted position alignment is effective for fine alignment between the lattice layers for functioning as diffraction gratings, exclusive use of this alignment method alone may result in a decrease in alignment efficiency. In contrast, first performing rough alignment while looking at the observation light lattice image (image of a lattice structure), and subsequently performing fine alignment (fine position adjustment) using the laser beam diffraction image, makes it possible to efficiently achieve precise position alignment between the lattice layers.
A further feature of the manufacturing method of the invention is that each lattice layer of the semiconductor wafers to be stacked is formed with a structure including a ordinary crystal lattice region and an adjustment lattice region, wherein the latter is greater in lattice period than the former. Providing the adjustment lattice region with greater lattice period than that used for photonic crystals facilitates the position alignment using the lattice image. Preferably, such adjustment lattice region is provided at specified part such as for example an edge portion of lattice layer since this region is the one that does not constitute any photonic crystals.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.